Buildings are usually designed with a space between a drop ceiling and a structural floor from which the ceiling is suspended to serve as a return air plenum for elements of heating and cooling systems as well as serving as a convenient location for the installation of communications cables and other equipment, such as power cables and data cables. Such data cables also called plenum cables. Alternatively, the building can employ raised floors used for cable routing and plenum space. Communications cables generally include voice communications, data and other types of signals for use in telephone, computer, control, alarm, and related systems, and it is not uncommon for these plenums and the cables therein to be continuous throughout the length and width of each floor, which can introduce safety hazards, both to the cables and the buildings.
When a fire occurs in an area between a floor and a drop ceiling, it may be contained by walls and other building elements which enclose that area. However, if and when the fire reaches the plenum space, and especially if flammable material occupies the plenum, the fire can spread quickly throughout the entire floor of the building. The fire can travel along the length of cables which are installed in the plenum if the cables are not rated for plenum use, i.e., do not possess the requisite flame and smoke retardation characteristics. Also, smoke can be conveyed through the plenum to adjacent areas and to other floors with the possibility of smoke permeation throughout the entire building.
As the temperature in a non-plenum rated jacketed cable rises, charring of the jacket material begins. Afterwards, conductor insulation inside the jacket begins to decompose and char. If the charred jacket retains its integrity, it still functions to insulate the core; if not, however, it ruptures due either to expanding insulation char or to pressure of gases generated from the insulation, and as a consequence, exposes the virgin interior of the jacket and insulation to the flame and/or the elevated temperatures. The jacket and the insulation begin to pyrolize and emit more flammable gases. These gases ignite and, because of air drafts in the plenum, burn beyond the area of flame impingement, thereby propagating flame thru the building and generating smoke and toxic and corrosive gases.
Because of the possibility of flame spread and smoke evolution, as a general rule, the National Electrical Code (NEC) requires that power-limited cables in plenums be enclosed in metal conduits. However, the NEC permits certain exceptions to this requirement. For example, cables without metal conduits are permitted, provided that such cables are tested and approved by an independent testing agent, such as Underwriters Laboratories (UL), as having suitably low flame spread and smoke generating or producing characteristics. The flame spread and smoke production of cables are measured using the UL 910 (1998 edition) or NFPA 262 (2011 edition), also known as the “Steiner Tunnel,” standard test method for fire and smoke retardation characteristics of electrical and optical fiber cables used in air handling spaces, i.e., plenums.
A riser (CMR) rated cable is a cable that meets the UL1666 requirements. These cables are designed for installations in vertical trays between floors or through elevator shafts. The cable typically contains insulated wires twisted, and jacketed with a Riser type jacket. The purpose of this invention is to describe a composition of a PVC compound used as a jacket for these types of cables.
The most important property of a CMR cable is to pass the vertical burn UL1666 test. The test apparatus and procedure is detailed in UL1666, section 4. The test is done by installing finished cables in a chamber. The cables are then exposed to a continuous flame (at 154.5 KW) for 30 minutes. To pass the test, the flame propagation can not equal or exceed 12 feet beyond the ignition point, and the temperature can not exceed 850° F. as described in section 9 of UL1666.
One of the main obstacles to developing a polyvinyl chloride (PVC) jacket composition to meet the UL1666 specifications, is to maintain a low flammability during the 30 minute burn. To do so, formulators tend to add a significant amount of bromine (in the form of brominated phthalates) and antimony. Those two additives, along with aluminum trihydrate, can provide for low flammability. However, those additives are costly.
Therefore, there remains a need for jacket compositions for plenum and riser cable that provide low flammability, but can be made inexpensively.